GLOST FIRING (Pottery, for artists)
CHAPTER XI GLOST FIRING "When Fortune bringeth thee affliction, console thyself by remembering that one day thou must see pros- perity, and another day difficulty." From EL KORAN. THE hard porous biscuit shape will now need a coat of glaze and a subsequent glost fire. Raw glazes for green shapes are now seldom used except for the coarser wares or peasant pottery. In commerce it is, however, largely used on tiles, mouldings, and big sanitary appliances. For com- mon cheap crockery a soft lead glaze, often galena, is generally used. It is applied with a brush, or the pot still leathery and tough is dipped in the glaze. Raw glazes have a strong inclination to leave in the firing. They are very difficult to manipulate unless they are of the simplest formula and fired very slowly. The addition of a little clay in the form of slip to the glaze will often counteract the tendency to leave the pot. The ordinary glazes in dry powder form are mixed with water to the consistency of cream and passed with the aid of a stout brush through a phosphor 107 108 bronze sieve into a large basin or tub. The sieves may range from eighty to two hundred mesh, accord- ing to the delicacy of the work. For ordinary work No. 100 suffices. The biscuit to receive the glaze should be quite clean and free from dust or dirt with the insides carefully dusted or blown out. Grease will stop absorption, but with opaque glazes discoloration is not of great importance. If the biscuit is hard and inclined to be non-porous, the glaze will need to be mixed fairly thick before it will cling; with soft and porous shapes a com- paratively thin mixture will take readily. No immutable law can be laid down for the exact thickness of the coat of glaze. All glazes vary. One sixteenth inch may be enough for one and far too thin for another. From one twentieth inch for thin trans- parent glaze to one eighth, inch for matts is a fair average. It is well to err on the thick side to avoid an impoverished look. Trials on odd pieces of biscuit fired in horizontal and vertical positions will best settle the point. In all cases the inside is first half filled with glaze which is rolled quickly round and out. The deposit is then tested with a knife. The glaze for the inside should be slightly thinner in composition than for the outside, as it is inclined to pool in the bottom if too thick. The outside and the neck are then scraped free of all splashes. 109 POUKINC FIG. 50 110 To glaze the outside of the pot the methods employed are painting, spraying, dipping, and pour- ing. In painting, two or three coats applied with a flat soft brush may be necessary. In spraying, the glaze is thinned considerably and blown through a vaporizer by means of a foot pump. (Fig. 51.) The shape, its mouth closed with a small bowl or saucer, is placed on a whirler in a draught box and slowly revolved. The draught draws the waste spray away from the operator, who should wear a respirator. This method is excellent where any gradation is required. It is easily learned. Un- fortunately, it requires expensive apparatus to ren- der it safe with lead glazes. Unless it is done on a big scale and the waste glaze retained, it is also wasteful. In dipping, the pot is plunged right under in a tub of glaze which is kept well stirred to prevent the heavy constituents settling. (Figs. 52 and 53.) This requires considerable dexterity. In a school, glaze is seldom mixed in sufficient quantities to per- mit of this being acquired. Perhaps the most satisfactory method is pouring. (Fig. 50.) The shape is reversed and stood upon a big stilt or two strips of wood over a bowl or tub. Then the glaze is poured from a jug round the edge of the base, until every part is covered. With a little practice a quite even coat can be thus applied. Ill yp LAYING FIG. 51 Owing to the inversion of the shape it is slightly thicker at the shoulder and neck, an excellent point in glazing. When dry, the base is scraped and wiped clean with a sponge and the lip touched up with a brush. With matt glazes, as has been noted, a thick coat is necessary. If very porous, the shape may be soaked first in clean water to take some of the suction out of the biscuit. When glazed, it is essential that all dips and inequalities be removed. If thinly coated, the glaze will assume a glossy surface when fired. PIPPING. FIG. 52 Whatever process is used, the lip and base of a pot should receive careful attention, the rims es- pecially should be thickly and evenly coated with glaze. The thickness can be tested with the point of a knife and any scratches or bare spots should be filled in and rubbed flat as they are apt to show at the finish if the glaze is at all refractory. Never starve the glaze, for a thin coat imparts a poor cheap look to any pot that is seldom remedied with entire success. When only one kind of glaze is used, the application is quickly learned. With hard and soft, lead and leadless glazes, both opaque and transparent, and possibly a combination of spraying, painting, and pouring, the difficulties are greatly increased. Prac- tical experience will be the only safe guide. But speaking generally, a sprayed coat can be thicker than 113 DIPPING a poured coat. Hard glazes give less trouble than soft if too liberally applied. Tin glazes will stand a lot of over-firing even when thin and the reverse holds good of matt glazes. Raw borax glazes require much more careful firing than raw lead glazes but often give better colour results and are less liable to sulphuring. When glazing with a transparent glaze over under- glaze painting a thin coat is advisable. If it be thick and run, the painting is spoiled ; but if, after firing, it appears thin, another light spray may be tried. But whole chap- ters of writing will never settle these points. Re- peated trials on odd pieces of biscuit will elucidate more than many books. In pack- ing a glost or glaze kiln stilts and spurs instead of saddles must be used to prevent the pots from stick- ing to the floor or shelves. (Figs. 49 and 54.) For the same reason the pots must not touch each other. With glazes requiring a high temperature the space between pots of different colour should be considerable. An appreciable amount of glaze is liberated in the in- tense heat and a blue pot will often leave a distinct blue patch on any light pot placed too near. Soft FIG. 53 114 ao<^)^ THIMBU* 5ocKET EGG STILT CONE-BITS FIG. 54 glazes liable to run or drip should be placed on separate bats well flinted beneath. Any crack in the muffle should be well stopped and lime may be used to neutralize the effects of any sulphur that may enter. As the back is usually hotter than the front the hard glazes should be packed first, and by selecting suitable shapes a good setter will pack a surprising amount into even a small kiln. The clamming should be done with care, as bits are liable to flake off and stick to the glaze. The top plug only need be left out and the lighting up proceeded with as before. The gradual increase of heat is very essential in firing glaze, for any sudden jump or reduction will play all manner of games in a glost oven. The 115 slow start is not so important, there being no risk of blowing. Indeed the glaze is best in a sharp quick fire. Although cones should always be used and are excellent indicators, experience will soon show when a kiln is fired up. It may be advisable to go on a little after the cone is over until all looks fluxed so as to get rid of bubbles. These appear in some glazes until the last. They do not always go down on cooling, but those glazes that bubble through over-firing should be avoided. Possibly the trouble will arise from sulphur in the body which must be corrected. Trials are always useful but they must be small and easily hooked out, as constant poking about at the spy hole lowers the temperature of the muffle at the front, just where it is most difficult to get it up. When fired, a glost kiln should cool very slowly, for any sudden access of cold air is liable to stunt or crack the pots. When firing over glaze decoration where the glaze does not run, the packing is much simplified. With low-firing lustres the pots may even touch each other. All this has been set out at great length but we shall find over and above this that each kiln has its own little ways which must be studied and hu- moured before the best results can be obtained. The chief thing to avoid is a sudden flush or jump in the heat. The main thing to aim at is a slow start steadily increasing to a sharp finish. The state of 116 the muffle, a dry or a damp day, will modify each firing a little, but the above generalization will have a fairly wide application to the working of a small kiln. Unpacking is a compound of despair and delight, and is best done slowly. As the colour dies the clam- ming, if any, may be knocked away, and later on the spy plugs removed. When comparatively cold, the door may be opened slightly, and left so for one or two hours. Then the front pots can be extracted but those at the back should be allowed another half hour. Be very careful of the razor edges of broken stilts or glaze dips. A steel chisel should be used for chopping rough edges or removing refractory stilts. If necessary the bases may be ground on an emery wheel until the pot stands true. Categoría:Pottery, for artists, craftsmen & teachers